90 lines
2.6 KiB
Markdown
90 lines
2.6 KiB
Markdown
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# Rail Fence Cipher
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Implement encoding and decoding for the rail fence cipher.
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The Rail Fence cipher is a form of transposition cipher that gets its name from
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the way in which it's encoded. It was already used by the ancient Greeks.
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In the Rail Fence cipher, the message is written downwards on successive "rails"
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of an imaginary fence, then moving up when we get to the bottom (like a zig-zag).
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Finally the message is then read off in rows.
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For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE",
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the cipherer writes out:
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. E . R . D . S . O . E . E . F . E . A . O . C .
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. . A . . . I . . . V . . . D . . . E . . . N . .
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```
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Then reads off:
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```text
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WECRLTEERDSOEEFEAOCAIVDEN
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```
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To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.
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```text
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? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
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. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
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. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
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```
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The first row has seven spots that can be filled with "WECRLTE".
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
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. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
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```
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Now the 2nd row takes "ERDSOEEFEAOC".
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. E . R . D . S . O . E . E . F . E . A . O . C .
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. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
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```
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Leaving "AIVDEN" for the last row.
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. E . R . D . S . O . E . E . F . E . A . O . C .
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. . A . . . I . . . V . . . D . . . E . . . N . .
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```
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If you now read along the zig-zag shape you can read the original message.
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* * * *
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For installation and learning resources, refer to the
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[Ruby resources page](http://exercism.io/languages/ruby/resources).
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For running the tests provided, you will need the Minitest gem. Open a
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terminal window and run the following command to install minitest:
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gem install minitest
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If you would like color output, you can `require 'minitest/pride'` in
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the test file, or note the alternative instruction, below, for running
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the test file.
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Run the tests from the exercise directory using the following command:
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ruby rail_fence_cipher_test.rb
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To include color from the command line:
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ruby -r minitest/pride rail_fence_cipher_test.rb
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## Source
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Wikipedia [https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher](https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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